Glycosaminoglycans, or mucopolysaccharides, along with collagen, are the chief structural elements of all connective tissues. Glycosaminoglycans, or gags, are large complexes of polysaccharide chains associated with a small amount of protein. These compounds have the ability to bind large amounts of water, thereby producing a gel-like matrix that forms the body's connective tissues. Gags are long chains composed of repeating disaccharide units (aminosugar-acidic sugar repeating units). The aminosugar is typically glucosamine or galactosamine. The aminosugar can also be sulfated. The acidic sugar may be D-glucuronic acid or L-iduronic acid. In vivo, gags other than hyaluronic acid are covalently bound to a protein, forming proteoglycan monomers. The polysaccharide chains are elongated by the sequential addition of acidic sugars and aminosugars.
Among the most common gags are hyaluronic acid, keratan sulfate, chondroitin sulfate, heparin sulfate, and dermatin sulfate. Gags may be chemically modified to contain more sulfur groups than in their initially extracted form. In addition, gags may be partially or completely synthesized and may be of either plant or animal origin.
Hyaluronic acid is a naturally occurring member of the glycosaminoglycan family which is present in particularly high concentration in the cartilage and synovial fluid of articular joints, as well as in vitreous humor, in blood vessel walls, and umbilical cord and other connective tissues. Hyaluronic acid can be in a free form, such as in synovial fluid, and in an attached form, such as an extracellular matrix component. This polysaccharide consists of alternating N-acetyl-D-glucosamine and D-glucuronic acid residues joined by alternating beta-1,3-glucuronidic and beta-1,4-glucosaminidic bonds. In water, hyaluronic acid dissolves to form a highly viscous fluid. The molecular weight of hyaluronic acid isolated from natural sources generally falls within the range of 5×104 up to 107 daltons. Hyaluronic acid has a high affinity for the extracellular matrix and to a variety of tumors, including those of the breast, brain, lung, skin, and other organs and tissues.
A drug delivery system is used for maintaining a constant blood level of a drug over a long period of time by administering a drug into the body, or for maintaining an optimal concentration of a drug in a specific target organ by systemic or local administration, and over a prolonged period of time.
Chemically modified hyaluronic acid can be used for controlled release drug delivery. Balazs et al, in U.S. Pat. No. 4,582,865, state that “cross-linked gels of hyaluronic acid can slow down the release of a low molecular weight substance dispersed therein but not covalently attached to the gel macromolecular matrix.”
Various forms of pharmaceutical preparations are used as drug delivery systems, including the use of a thin membrane of a polymer or the use of a liposome as a carrier for a drug.
There are two basic classes of drug carriers: particulate systems, such as cells, microspheres, viral envelopes, and liposomes; and non-particulate systems, which are usually soluble systems, consisting of macromolecules such as proteins or synthetic polymers.
Generally, microscopic and submicroscopic particulate carriers have several distinct advantages. They can perform as sustained-release or controlled-release drug depots, thus contributing to improvement in drug efficacy and allowing reduction in the frequency of dosing. By providing protection of both the entrapped drug and the biological environment, these carriers reduce the risks of drug inactivation and drug degradation. Since the pharmacokinetics of free drug release from the particles are different from directly-administered free drug, these carriers can be used to reduce toxicity and undesirable side effects.
Despite the advantages offered, there are some difficulties associated with using drug encapsulating biopolymers. For example, biopolymers structured as microparticulates or nanoparticulates have limited targeting abilities, limited retention and stability in circulation, potential toxicity upon chronic administration, and the inability to extravasate. Numerous attempts have been made to bind different recognizing substances, including antibodies, glycoproteins, and lectins, to particulate systems, such as liposomes, microspheres, and others, in order to confer upon them some measure of targeting. Although bonding of these recognizing agents to the particulate system has met with success, the resulting modified particulate systems did not perform as hoped, particularly in vivo.
Other difficulties have also arisen when using such recognizing substances. For example, antibodies can be patient-specific, and thereby add cost to the drug therapy. Additionally, not all binding between recognizing substrate and carrier is covalent. Covalent bonding is essential, as non-covalent binding might result in dissociation of the recognizing substances from the particulate system at the site of administration, due to competition between the particulate system and the recognition counterparts to the target site for the recognizing substance. Upon such dissociation, the administered modified particulate system can revert to a regular particulate system, thereby defeating the purpose of administration of the modified particulate system.